Game modes. I especially miss the old style rumble pit with Oddball, Juggernaut, and KotH. Fiesta, Infection, and Assault would be nice too. I'm really puzzled at the omission of assault since they seem to have most of the assets for it in the game in Warzone Assault, and also since Breakout should be assault (neutral bomb) rather than CTF. CTF = take enemy's flag back to your base, assault = take a bomb to the enemy base, right? If they brought that back, Grifball could come back for the crowd that wants that, though they'd also have to add the gravity hammer.
Besides game modes and some maps, though, I'm hoping they get more creative with the vehicles too. The better vehicle reqs are mostly basically the same vehicle with better armor. How about a Wraith where the secondary turret shoots needler ammo? Why isn't there a scorpion that lets the driver use the machine gun turret too, without a gunner? Why not a ghost with a higher top speed and the speed boost replaced with a decent jump? Can't the UNSC modify a Gungoose to use non-frag grenade ammo? (Granted, getting used to one that fires splinter grenades would take a while until you stop EMPing yourself.) Anything like that would be more fun than the progression of Warthog -> Warhog with better armor -> Warhog with EVEN BETTER armor -> Warhog with that EVEN BETTER armor and SNOW CAMO.